The present disclosure generally relates to displaying content to a user wearing a head-mounted display (HMD) as part of an artificial reality system, and specifically relates to a dynamic control of optical axis location in HMDs.
Vergence-accommodation conflict is a phenomenon that occurs to users of virtual headsets such as HMDs. Typically, eyes converge (rotate toward one another) to focus on closer objects and diverge (rotate away from one another) to focus on objects that are further away. The vergence therefore represents the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. Accommodation is coupled with vergence, and is the process where the lenses of the eyes focus on a close or far away object. During accommodation of an eye, a crystalline lens of the eye changes optical power to maintain a clear image or focus on an object as the object's distance varies. In HMD systems, vergence and accommodation processes are decoupled. In fixed-focused HMD systems, the user's eyes verge to a virtual object, but the accommodation stimulus is incorrect for near objects. The eyes may accommodate to the fixed focus distance of a display in the HMD, conflicting with the verged distance to the virtual object. More often the eyes will accommodate for a near object, which causes image blur since the virtual object distance is fixed. The decoupling of vergence and accommodation processes can cause the user to feel uncomfortable, disoriented, or nauseous. Additionally, in some artificial reality systems (e.g., in augmented reality systems), there is a location mismatch between a real-world object and an augmented reality image.
It is difficult for conventional liquid crystal lenses to meet all required design specifications on an aperture size, an optical power, a switching speed, and an image quality to mitigate the vergence-accommodation conflict and eliminate the location mismatch. For example, a large aperture size (e.g., larger than 4 cm) can be required for achieving a large field-of-view in an artificial reality system; a large optical power (e.g., ±2D) can be required for adapting human eye vergence-accommodation; a fast switching speed (e.g., approximately 300 ms) can be required for adapting human eye vergence-accommodation; and good image quality can be required for meeting human eye acuity. However, the trade-offs among the aperture size, switching speed, accommodation power and image quality is hard to resolve for the conventional liquid crystal lenses.